1. Field of the Invention
The present invention relates to an ink jet recording sheet. More particularly, the present invention relates to an ink jet recording sheet having an excellent gloss and a good ink jet recording ability.
2. Description of the Related Art
The recording system using an ink jet printer is widely utilized in various fields due to the low printing noise, the high printing speed and easy full color printing. As an ink jet recording sheet, woodfree paper sheets having a high ink-absorbing property or coated paper sheets having a porous pigment-containing coating layer formed on a substrate paper sheet, are used. However, almost all of the conventional ink jet recording sheets have a low surface gloss and thus a mat sheet-like appearance. Therefore, there is a strong demand to new type of ink jet recording sheets having a high surface gloss and an excellent appearance.
Generally, as a high gloss recording sheet, a high gloss coated sheet prepared by coating a substrate sheet surface with a plate-crystalline pigment and optionally calendering, or a cast-coated sheet prepared by press-casting a wetted coating layer formed on a substrate sheet onto a mirror-finished periphery surface of a casting drum and drying the press-casted coating layer to transfer the mirror-finished surface of the casting drum to the coating layer, are known.
The cast-coated sheets have higher surface gloss and smoothness than those of the conventional calendered coated sheets, and thus exhibit an excellent printing effect. Therefore, the conventional cast-coated sheets are mainly used for high gloss prints. When used for the ink jet printing, the conventional cast-coated sheets cause various difficulties.
Conventional cast-coated sheets are disclosed in, for example, U.S. Pat. No. 5,275,846 for T. Imai et al. In the conventional cast-coated sheets, on one hand, a film-forming substance, for example, a binder, contained in a pigment composition for forming a coating layer, serves to transfer a mirror-finished casting surface of a casting coater drum to the coating layer, and the resultant cast-coated layer has a high gloss. On other hand, the film-forming substance causes, for example, the resultant cast-coated layer to exhibit a reduced porosity and a significantly reduced absorption of the ink in the ink jet printing procedure. To enhance the ink absorption, it is important that the cast-coated layer is porous so as to enables the layer to easily absorb the ink. For this purpose, it is necessary to reduce the content of the film-forming substance in the cast-coated layer. The reduction in the content of the film-forming substance causes the white sheet gloss of the resultant cast-coated layer to decrease. As mentioned above in detail, it is practically very difficult to obtain a cast-coated layer satisfactory in both the surface gloss and ink jet printing ability.
As means for solving the above-mentioned problems, Japanese Unexamined Patent Publication No. 7-89,220, for the inventors of the present invention, discloses a cast-coated paper sheet, useful as an ink jet recording sheet, produced by forming a recording layer containing a pigment and a binder on a substrate paper sheet; coating a coating layer comprising, as a principal component, a copolymer composition produced by copolymerizing comonomers each having an ethylenically unsaturated group and having a glass transition temperature of 40.degree. C. or more on the recording layer to form a coating liquid layer to be converted to a cast-coated layer; press casting the coating liquid layer onto a mirror-finished and heated casting surface of a drum, while the coating liquid layer is kept in a wetted condition; drying the press-casted coating liquid layer to form a cast-coated layer; and separating the casting surface from the resultant cast-coated layer. It was found that the resultant ink jet recording sheet has both a high gloss and an excellent ink-absorbing property.
However, due to significant enhancements in recording speed of ink jet recording procedure and minuteness and accuracy of the ink jet printed images, and an extensive improvement in full color printing technology, the ink jet recording sheet is strongly required to exhibit an enhanced gloss, improved sharpness and clarity, and high color density of ink images. For example, the ink jet recording sheet is required to exhibit high gloss and image quality comparative to those of the conventional silver salt-type photographic paper sheet. The above-mentioned prior art ink jet recording sheets cannot satisfy the new requirements.